PHP preg_match Examples

Use preg_match() to match strings with regular expressions. Check the return value for true to see if the expression did match.

Syntax of preg_match

While full syntax is

 int preg_match ( string $pattern , string $subject 
      [, array &$matches [, int $flags = 0 [, int $offset = 0 ]]] )

you propably will use preg_match() mostly with two parameters for simply matching checks or with three to extract matches.

You probably won’t use the 4th and 5th parameter which can be used to return match offsets and limit matching to a given offset in the string.

Simple String Checks

Here are some syntax examples that check strings for certain content:

Basic matching

 preg_match("/PHP/", "PHP")       # Match for an unbound literal
 preg_match("/^PHP/", "PHP")      # Match literal at start of string
 preg_match("/PHP$/", "PHP")      # Match literal at end of string
 preg_match("/^PHP$/", "PHP")     # Match for exact string content
 preg_match("/^$/", "")           # Match empty string

Using different regex delimiters

 preg_match("/PHP/", "PHP")                # / as commonly used delimiter
 preg_match("@PHP@", "PHP")                # @ as delimiter
 preg_match("!PHP!", "PHP")                # ! as delimiter

Changing the delimiter becomes useful in some cases

 preg_match("/http:\/\//", "http://");     # match http:// protocol prefix with / delimiter
 preg_match("#http://#",   "http://")      # match http:// protocol prefix with # delimiter

Case sensitivity

 preg_match("/PHP/", "PHP")                # case sensitive string matching
 preg_match("/php/i", "PHP")               # case in-sensitive string matching

Matching with wildcards

 preg_match("/P.P/",     "PHP")            # match a single character
 preg_match("/P.*P/",    "PHP")            # match multipe characters
 preg_match("/P[A-Z]P/", "PHP")            # match from character range A-Z
 preg_match("/[PH]*/",   "PHP")            # match from character set P and H
 preg_match("/P\wP/",    "PHP")            # match one word character
 preg_match("/\bPHP\b/", "regex in PHP")   # match the word "PHP", but not "PHP" as larger string

Using quantifiers

 preg_match("/[PH]{3}/",   "PHP")          # match exactly 3 characters from set [PH]
 preg_match("/[PH]{3,3}/", "PHP")          # match exactly 3 characters from set [PH]
 preg_match("/[PH]{,3}/",  "PHP")          # match at most 3 characters from set [PH]
 preg_match("/[PH]{3,}/",  "PHP")          # match at least 3 characters from set [PH]

Extracting Data

To extract data using regular expression we have to use capture/grouping syntax.

Some examples:

 # Extract everything after the literal "START"
 preg_match("/START(.*)/", $string, $results)   

 # Extract the number from a date string 
 preg_match("/(\d{4})-(\d{2})-(\d{2})/", "2012-10-20", $results)

 # Nesting of capture groups, extract full name, and both parts...
 preg_match("/name is ((\w+), (\w+))/", "name is Doe, John", $results)

So you basically just enclose the sub patterns you want to extract with braces and fetch the results by passing a third parameter which preg_match() will fill as an array.

Named Capture Groups

 # Extract the number from a date string 
 preg_match("/(?P&lt;year>\d{4})-(?P&lt;month>\d{2})-(?P&lt;day>\d{2})/", "2012-10-20", $results)

Now the $result array will additionally to the position matches 1, 2 and 3 contain the keys “year”, “month” and “day”. The advantage is never having to think of the capture positions anymore when you modify the expression!

Check for Processing Errors!

While it might often be unimportant be aware that applying a regular expression might fail due to PCRE constraints. This usually happens when matching overly long strings or strings with faulty encoding.

The only way to notice that preg_match() was not able to check the string is by calling

 preg_last_error()

Only if it returns PREG_NO_ERROR you got a safe result! Consider this when using preg_match() for security purposes.

PHP preg_match Examples Edit Cheat Sheet